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Capillary Flow Experiment (CFE) - 11.22.16

Science Objectives for EveryoneCapillary Flow Experiment (CFE) is a suite of fluid physics experiments that investigate capillary flows and flows of fluids in containers with complex geometries. Results will improve current computer models that are used by designers of low gravity fluid systems and may improve fluid transfer systems on future spacecraft.

Science Results for Everyone
In space, you can’t just go with the flow. This is because it is not that easy to know where the flow is. Liquids simply do not collect at the bottom of the containers that hold them because there is no ‘bottom’ in space. Controlling the flow of fluids is difficult in microgravity, which hampers design of systems such as liquid propellants, thermal control, and waste-water management. But capillary forces, which draw fluids up a narrow tube, continue to act in microgravity and can control fluid orientation on spacecraft. This investigation examined capillary and fluid flows in three types of systems in microgravity. Investigators are compiling a database of videos, data, and preliminary results of all events. Results help to improve computer models used to design microgravity fluid systems and may improve fluid transfer systems on future spacecraft.

The following content was provided by Mark Milton Weislogel, Ph.D., and is maintained in a database by the ISS Program Science Office.

Without gravity, it is more difficult to control the flow of fluids and this is a challenge for designing spacecraft systems such as liquid propellants/cryogens, thermal control systems, wastewater management and recycling.

Capillary forces (the interaction of liquid with solid that can draw a fluid up a narrow tube) continue to act in the absence of gravity and can be exploited to control fluid orientation so that fluid systems on spacecraft perform predictably.

CFE uses the low-gravity environment provided by the International Space Station to understand the special dynamics of capillary flow and will aid in the design of fluid transport systems on future spacecraft.

Description
The Capillary Flow Experiment (CFEs) is a suite of fluid physics experiments whose purpose is to investigate capillary flows and phenomena in low gravity. The CFE data to be obtained will be crucial to future space exploration because they provide a foundation for physical models of fluids management in microgravity, including fuel tanks and cryogen storage systems, Thermal Control Systems (TCS) (e.g., water recycling), and materials processing in the liquid state. NASA's current plans for Exploration missions assume the use of larger liquid propellant masses than have ever flown before. Under low-gravity conditions, capillary forces can be exploited to control fluid orientation so that such large mission-critical systems perform predictably.

The handheld experiments common to the suite aim to provide results of critical interest to the capillary flow community that cannot be achieved in ground-based tests; for example, dynamic effects associated with a moving contact boundary condition, capillary-driven flow in interior corner networks, and critical wetting phenomena in complex geometries. Specific applications of the results center on particular fluids challenges concerning propellant tanks. The knowledge gained will help spacecraft fluid systems designers increase system reliability, decrease system mass, and reduce overall system complexity.

CFE encompasses three experiments, CFE-Contact Line (CFE-CL), CFE-Interior Corner Flow (CFE-ICF), and CFE Vane Gap (CFE-VG), with two unique experimental apparatuses per experiment. There are multiple tests per experiment. Each of the experiments employs conditions and test cell dimensions that cannot be achieved in ground-based experiments. All of the units use similar fluid injection hardware made of Lucite, have simple and similarly sized test chambers, and rely solely on video for highly quantitative data. Silicone oil is used as the fluid. Differences between units are primarily fluid properties, wetting conditions (determined by the coating inside the test chamber), and test cell cross section.

CFE-CL investigates the properties of the contact line (the boundary between the liquid and the solid surface of the container). The contact line controls the interface shape, stability, and dynamics of capillary systems in low gravity.

CFE-ICF studies capillary flow in interior corners. Structured inside tanks providing interior corners are used in the design of fuel tanks so that the fuel will always flow to the outlet of the tank in the absence of gravity. The equations governing the process are known but, to date, have not been solved analytically because of a lack of experimental data identifying the appropriate boundary conditions for the flow problem. Experimental results will guide the analysis by providing the necessary boundary conditions as a function of container cross section and fill fraction. The benchmarked theory can then be used to improve propellant management aboard spacecraft.

CFE-VG studies capillary flow when there is a gap between interior corners, such as in the gap formed by an interior vane and tank wall of a large propellant storage tank or the near intersection of vanes in a tank with complex vane network.

During each experimental run a crewmember will disturb the fluids by tapping the container, moving the vanes, etc. By digitizing and quantitatively analyzing video data of the resulting oscillations, natural frequencies and damping rates will be determined. The effects of partial wetting, the lag before contact angle changes, and fluid properties such as surface tension and viscosity will be quantified. Transient flow rates, stability limits, and coalescence time scales will be measured.
^ back to topApplications

Space Applications
The knowledge gained from this payload has the potential to be instrumental in the design of future fluid systems for spacecraft-impacting fluid bearing containers such as propellant and cryogenic fluids tanks, thermal control system coolant reservoirs, water storage and management systems, liquid state low-gravity materials processing equipment, and biofluids handling instruments for inflight human health systems. By performing this experiment, researchers will gain information that will lead to improvements in system reliability with reductions in system mass and complexity. These applications of CFE are in direct support of NASA's mission to develop safe, reliable, and affordable spacecraft to pursue the greater exploration of our solar system and universe.

Earth Applications
The results of the flight experiments are also expected to provide insights into terrestrial interfacial phenomena and may lead to models predicting fluid flows in porous media (i.e. ground water transport), complex capillary structures (i.e. high performance wicks for heat pipes employed in electronics cooling), and Lab-On-Chip technologies (i.e., microscale biofluids processing).

Operational Requirements and Protocols
During the CL experiments, crewmembers will allow the fluids to settle out for approximately 20 min. The crew will then impart disturbances by tapping, pushing, sliding, swirling, and shaking the CFE unit. The crew will start and stop the camcorder and change/label tapes as required to record all tests. Following each disturbance, the crew will allow the fluids to dampen before proceeding to the next disturbance.

For the VG sessions, crewmember will fills the test chamber with fluid and then record the background g-jitter during an undisturbed wait with the ISS camcorder. For VG-2, crewmembers will then increment the vane through one complete revolution capturing the advance and recession of the fluid along each side of the vane. The critical angles at which the fluid spontaneously rises to the top of each side the vane are also determined and recorded. This test is special in that the surface is dry and is not repeatable. During VG-1, crewmembers will increment the vane through two complete revolutions capturing the advance and recession of the fluid along each side of the vane. The critical angles at which the fluid spontaneously rises to the top of each side the vane are also determined and recorded. This test sees a wet surface in the test chamber and vane and may be repeated as many times as necessary.

For the ICF experiments, the crewmember injects a fluid from a self-contained reservoir into the test chamber and primes the tube between valve 2 and the test chamber vertex. The crewmember will then turn the knob and open/close valves 1 or 2 to dispense or retract fluid into test chamber. Fluid creeps from test chamber base to top vertex and is then recovered to a reservoir.The crew will set up the Maintenance Work Area (MWA) work surface and camcorder, attach the CFE units onto the MWA, inject the fluids into the test chambers, and record the fluid's response to disturbances using the ISS camcorder.

The Capillary Flow Experiment (CFE) is a space fluid physics study of capillary flow in a complex geometry using hand-held experimental devices. CFE consists of six approximately 1–2kg experiment hand-held units designed to study capillary flow in complex containers demonstrating the capillary phenomena of Interior Corner Flow (ICF), Vane Gap (VG), and Contact Line (CL). High-resolution quality video images from the very simple to conduct experiments provide direct confirmation of the usefulness of current analytical design tools as well as provide guidance to the development of new ones. Results from these experiments are used to develop more accurate fluid models to aide in the design of low gravity fluid systems and enhance the fluid transfer systems of future space vehicles.

Interior Corner Flow (ICF)

Results CFE-ICF studies capillary flows along the tapering interior corners of two containers. The ullage (empty part of the container) migration rates of five different flows (dry, wet, open loop, closed loop, and bubbly flows) in the units were observed and compared to theoretical predictions. Migration rates were found to be in surprising agreement with predictions for the dry tests, but were under-predicted for previously wetted surfaces which are common in such systems. This analysis is ongoing and will be completed in part with further experimental results to be collected during CFE-2 experiments currently onboard ISS. In many cases bubbles are separated passively due to the specific geometry of the containers providing a no-moving-parts solution to this low-gravity challenge.

Vane Gap (VG)

Results The Capillary Flow Experiment-Vane Gap (CFE-VG) studies capillary flow when there is a gap between interior corners, such as in the gap formed by an interior vane and tank wall of a large propellant storage tank or the near intersection of vanes in a tank with a complex vane network. The CFE-VG experiment highlights the sensitivity of a capillary fluid surface to container shape and how small changes to the shape may result in dramatic global shifts of the liquid within the container. Understanding such behaviors is central to the passive management of liquids aboard spacecraft and in certain cases permits us the ability to move (pump) large quantities (potentially tons) of liquid by a simple choice of container shape. In particular, the Vane-Gap experiments identify the critical geometric wetting conditions of a vane structure that does not quite meet the container wall—a construct arising in various fluid systems aboard spacecraft such as liquid fuel and cryogen storage tanks, thermal fluids management, and water processing equipment. Critical vane gap wetting, as demonstrated by the flight experiments, is dependent on the vane angle and fluid wetting properties, similarly as for ideal sharp corners, but also by the additional geometric effects of the specific vane gap, vane thickness, container size and shape, and even time. Concerning applications, and in concert with previous investigations, it is evident that fluids may be positioned and/or otherwise controlled as desired by simply and slightly changing the geometry of container. In the case of the perfectly wetting fluid of VG-1, six wetting and de-wetting configurations were produced: wetting along the small gap between the vane and cylinder, wetting along the large gap, and a large shift in fluid from one side of the container to the other (bulk shift). De-wetting of all these conditions is also studied. The most noteworthy condition was the bulk shift due to the considerable amount of liquid that was transferred. We recorded large and small gap critical wetting angles (vane angles at which fluid draws up the gap formed between the vane and cylinder wall and “wets” the entire length of the vane) that were in close agreement with analytical predictions. However, the bulk shift phenomena was not predicted by analysis, which assumed perfect geometric symmetry of vane and cylinder. The occurrence of bulk shift indicates that small irregularities in geometry (such as tiny unavoidable imperfections within the manufacturing tolerance of the experiment unit) can influence fluid behavior in a significant way. Post flight measurements of test cell asymmetries were made to quantify such values and new predictions were made of the slightly asymmetric vessel. The theory was extended to address critical wetting events attributable to finite asymmetries verses infinitesimal ones. The agreement is informative as a method to assess tank symmetry for applications aboard spacecraft. For the partial wetting fluid of VG-2, critical wetting occurred along both the small and large gaps, similar to VG-1. However, unlike VG-1, the difference between the critical angles observed and predicted was substantial, and is likely due to asymmetry inherent in the experiment unit and contact angle hysteresis (the difference between the receding (uphill) angle and advancing (downhill) contact angle made by the fluid on a tilted plane right before the fluid drop begins to roll). Bulk shift of the fluid was not predicted and did not occur.

Contact Line (CL)

Results CFE-CL investigates the properties of the contact line (the boundary between the liquid, gas, and solid surface of the container), which controls the interface shape, stability, and dynamics of capillary systems in low gravity. From the over 400 events evaluated thus far, damping rate, frequency and qualitative waveform were found to be significantly influenced by contact line and contact angle conditions. The pinning condition produced higher frequencies and lower damping rates than the smooth condition. Larger contact angles also produced the same trend in higher frequencies and lower damping rates. Fluid depth was found to have little effect on the fluid response to disturbances except in cases where shallow tests were performed intentionally (Weislogel 2008). Observed results were also compared to model predictions. Agreement between numerical predictions and observed results varied widely. The most accurately modeled instance was the perfectly wetting case of CL-2 for both the pinning and smooth conditions and for both axial and lateral disturbance events. The least accurately modeled instances are the high contact angle push-type disturbance events with free-slip conditions, since contact line translation is not included in the constant contact angle slip model. In general, modeled and observed results were in best agreement for pinned conditions, as a result of the more predictable and confined contact line movement.

Phase Separation

Capillary solutions have long existed for the control of fluids in spacecraft fluid systems such as liquid propellants, cryogens and thermal fluids for temperature regulation. Such large length scale, ‘low gravity,’ capillary systems make use of container shape and fluid properties—primarily wetting—to passively locate or transport liquids to desired positions for a variety of purposes. Such methods have only been confidently established if the wetting conditions are known and favorable. However, capillary solutions for water-based systems aboard spacecraft are often ignored due to poor and unpredictable wetting properties. Application of CFE findings to spacecraft fluid systems is direct (Weislogel et al. 2009). In general, it is observed that with air-liquid flow rate ratios ≈ 100, and provided the capillary air capture section of the device is not flooded with liquid, 100% separation can be achieved independent of the wetting conditions tested, i.e. zero liquid carryover in air exit lines. This level of performance is ideal. It is also particularly encouraging for capillary solutions applied to poor wetting systems such as water in plastic containers. For the highly wetting fluids the vane structure and general geometry of the test device work well to collect all of the liquid by air drag and significant capillary driven flow, but the wetting and spreading nature of the liquid leads to thin films on nearly all, if not all, interior solid surfaces. Eventually, these films migrate towards the air exit line aided by the air flow. To prevent liquid carryover in such cases a simple feature is desired to pin the advancing films in a manner that forces the liquid into a recirculation rather than carryover. A ‘stove pipe’ geometry is employed to this effect. Liquid dragged by air flow builds at this edge whereupon it makes a significant capillary connection with liquid contained in the vane structure and is wicked away until the vane structure itself is saturated. Provided the vane structure saturation conditions are not exceeded, with continuous air flow, the liquid in the films simply recirculates near the stovepipe without liquid carryover into the air exit lines. This design uses capillary force to hold the liquid together in a continuous connected piece for efficient removal. With further development and modeling efforts along these lines, a new family of reliable and repeatable design options for the passive control of partially wetting systems may soon be available for evaluation and/or application in spacecraft fluid systems.

NASA Image ISS012E12911 - Astronaut William S. (Bill) McArthur Jr., Expedition 12 commander and NASA space station science officer, prepares the Capillary Flow Experiment (CFE) for video documentation. The CFE was positioned on the Maintenance Work Area in the Destiny laboratory of the International Space Station.+ View Larger Image

NASA Image ISS015E10587 View of Capillary Flow Experiment (CFE) in the U.S. Laboratory/Destiny. The purpose of this experiment is to investigate capillary flows and phenomena onboard the International Space Station (ISS). A video camera is set up to record the behavior of silicone oil in the Contact Line (CL) unit. Photo was taken during Expedition 15.